1. Field of the Invention
The present invention relates to a radiation image data reading apparatus that obtains image signals which represent radiation image data by reading the phosphorescent light emitted from a stimulable phosphor sheet, having said radiation image data recorded thereon, when irradiated by an excitation light. In particular, the present invention relates to a radiation image data reading apparatus that reads radiation image data from a stimulable phosphor sheet housed in a cassette.
2. Description of the Related Art
There are known stimulable phosphors that store a portion of radiation energy when irradiated by radiation. These phosphors, when irradiated by an excitation light such as visible light or laser light, emit phosphorescent light corresponding to said radiation energy stored therein. Radiation image recording/reproducing systems that utilize stimulable phosphor sheets constructed of these stimulable phosphors layered on a substrate are in wide practical use.
These radiation image recording/reproducing systems record radiation image data onto these stimulable phosphor sheets by irradiating thereon radiation that has been passed through a subject such as a human body. Afterwards, said sheet is two-dimensionally scanned by an excitation light such as a laser light, causing phosphorescent light to be emitted from the portion irradiation by said excitation light. Image signals which represent the aforementioned radiation image data are obtained by reading this phosphorescent light with photoelectric reading means (see, for example, Japanese Unexamined Patent Publication Numbers 55(1980)-12429, 55(1980)-116340, and 56(1981)-104645).
The image signals obtained by these systems are subjected to image processes appropriate for image observation and reading, such as a gradation process and a frequency process, etc. The radiation image represented by said image signals are then recorded as a visible image for diagnosis onto film, or displayed on a CRT display apparatus. If an erasing light is irradiated onto a stimulable phosphor sheet following radiation image data readout therefrom, the energy stored thereon is released, and the sheet is again in a state in which it can record radiation image data, and repeated use thereof becomes possible.
In many cases, the aforementioned stimulable phosphor sheets are handled while housed in light-shielding cassettes. These cassettes house said stimulable phosphor sheets one at a time, and usually constitute a box portion having an opening for inserting and removing said stimulable phosphor sheet, and a lid portion for opening and closing said opening.
When photographing (recording) a radiation image employing a cassette as described above, said cassette is placed in a position where it will be irradiated by radiation that has, for example, passed through a subject. By this positioning, the stimulable phosphor sheet within the cassette is irradiated by said radiation, and a radiation image is recorded thereon.
The cassette, after a radiation image has been recorded on the stimulable phosphor sheet therein, is set in a radiation image data reading apparatus as disclosed, for example, in U.S. Pat. No. 5,308,994. The stimulable phosphor sheet is taken out of the cassette by said apparatus, and is subjected to a readout process of the radiation image data.
With regard to the aforementioned radiation image data reading apparatus utilized in the radiation image recording/reading system, as disclosed in U.S. Pat. Nos. 4,346,295 and 4,485,302 and Japanese Unexamined Patent Publication No. 8(1996)-116435, detecting the phosphorescent light emitted from the stimulable phosphor sheet from both sides thereof is being considered. This is to improve the detection efficiency of the emitted phosphorescent light and to obtain image signals with a good signal to noise ratio. In this case, the excitation light may be irradiated on one or both sides of the stimulable phosphor sheet.
With regard to the aforementioned stimulable phosphor sheet, the use of a rigid sheet that is substantially inflexible is also being considered. These so-called rigid type stimulable phosphor sheets differ from flexible stimulable phosphor sheets in that they themselves can be moved directly, obviating the need for sheet conveyance means such as an endless belt.
Existing radiation image data reading apparatuses as described above that read radiation image data from a stimulable phosphor sheet housed in a cassette, as disclosed in aforementioned U.S. Pat. No. 5,308,994 are structured to read flexible stimulable phosphor sheets. That is, they are structured to convey a stimulable phosphor sheet, after removal from the cassette receiving portion, along a conveyance path to a readout portion, then perform a main scan with an excitation light while moving said sheet in a sub-scan direction, reading the radiation image data thereby.
It is conceivable to house the aforementioned rigid type stimulable phosphor sheet in the cassette described above. However, in that case it becomes impossible to move said stimulable phosphor sheet in a sub-scan direction while bending it at the readout portion. A necessity arises to provide one sheet""s worth of space in the readout portion along the sheet conveyance direction both before and after the main scan portion. Therefore, it was easy for radiation image data reading apparatuses that handled cassettes housing rigid type stimulable phosphor sheets therein to become excessively large.
The present invention has been developed in consideration of the circumstances described above, and it is an object of the present invention to miniaturize a radiation image data reading apparatus that handles cassettes housing rigid type stimulable phosphor sheets therein.
The radiation image data reading apparatus of the present invention realizes miniaturization of the apparatus by performing a sub-scan of a stimulable phosphor sheet by an excitation light while said sheet is being conveyed out from the cassette.
More specifically, the radiation image data reading apparatus of the present invention is a radiation image data reading apparatus that receives a cassette that houses a substantially inflexible stimulable phosphor sheet with high rigidity therein with an opening at an end portion thereof for inserting and removing said sheet having radiation image data recorded thereon and reads out radiation image data from said stimulable phosphor sheet which has been removed from said cassette comprising: a sheet conveyance means that conveys said stimulable phosphor sheet from the opening of said received cassette at a constant speed, at least during the readout of radiation image data; an excitation light main scan means that scans the stimulable phosphor sheet as it is being removed from said cassette with an excitation light in an excitation wavelength region of said sheet in a direction substantially perpendicular to the conveyance direction, at a position in the vicinity of said opening; and a photoelectric detection means that detects the phosphorescent light emitted from the portion of said sheet irradiated by the excitation light.
The aforementioned sheet conveyance means may change the conveyance speed at times other than that during which radiation image data is being readout. For example, it may be constructed to convey the stimulable phosphor sheet at a higher speed than the aforementioned constant speed until a front end of the stimulable phosphor sheet reaches the excitation light irradiation position.
With regard to the aforementioned excitation light main scan means of the radiation image data reading apparatus of the present invention, it is preferable that it be constructed of a line light source that irradiates the stimulable phosphor sheet with an excitation light in the form of a fan beam.
It is preferable that the aforementioned photoelectric detection means be constructed of line sensors. It is also desirable that the photoelectric detection means be constructed of a first photoelectric detection means that detects the phosphorescent light emitted from the portion of a stimulable phosphor sheet irradiated by the excitation light from one side of said sheet, and a second photoelectric detection means that detects said phosphorescent light from the other side of said sheet.
In addition, it is also desirable that the excitation light main scan means be constructed of a first excitation light main scan means that irradiates a stimulable phosphor sheet with an excitation light from one side thereof, and a second excitation light main scan means that irradiates said sheet from the other side thereof.
Meanwhile, with regard to the stimulable phosphor sheet to be used with the radiation image data reading apparatus of the present invention, it is desirable to utilize an anisotropic stimulable phosphor sheet that restricts the spreading of the excitation light and/or the phosphorescent light within said sheet.
Further, it is desirable that the radiation image data reading apparatus of the present invention be equipped with: a cassette holding portion for receiving a plurality of cassettes in a row; and a shift means for shifting the relative position between the sheet conveyance means, the excitation light main scan means as well as the photoelectric detection means and the aforementioned plurality of cassettes received in said cassette holding portion; wherein by shifting said relative position, it becomes possible to read out radiation image data from each of the stimulable phosphor sheets sequentially conveyed from said plurality of cassettes.
The radiation image data reading apparatus of the present invention is constructed to perform a main scan with an excitation light positioned near an insertion/removal opening of a received cassette when the rigid type stimulable phosphor sheet is being conveyed out of said received cassette while at the same time performing a sub-scan by said conveyance. Therefore, only one sheet""s worth of space needs to be provided along the sheet removal direction beyond the main scan portion, thereby making sufficient miniaturization possible.
Note that if the radiation image data reading apparatus of the present invention utilizes line light sources that irradiate a stimulable phosphor sheet with an excitation light in the form of a fan beam as its excitation light main scan means, it becomes further advantageous in miniaturization of the apparatus. This is because such a construction, when compared with a case in which a single deflected excitation light beam performs a main scan on a stimulable phosphor sheet, obviates the necessity of providing a large space for the deflection of the excitation light beam.
In addition, if the radiation image data reading apparatus of the present invention utilizes line sensors as the photoelectric detection means, further miniaturization of the apparatus becomes possible. This is because such a construction, when compared with a case in which a large light collector is utilized to collect the emitted phosphorescent light then said light is guided to a photoelectric detection means such as a photo multiplier, simplifies the construction of the phosphorescent emitted light photoelectric detection portion.
If the photoelectric detection means is constructed of a first photoelectric detection means that detects the phosphorescent light emitted from the portion of a stimulable phosphor sheet irradiated by the excitation light from one side of said sheet, and a second photoelectric detection means that detects said phosphorescent light from the other side of said sheet, the detection efficiency of said emitted phosphorescent light is improved and it becomes possible to obtain readout image signals with a good signal to noise ratio by adding the output signals of the two photoelectric detection means.
If the excitation light main scan means is constructed of a first excitation light main scan means that irradiates a stimulable phosphor sheet with an excitation light from one side thereof, and a second excitation light main scan means that irradiates said sheet from the other side thereof, the excitation efficiency is improved, and it becomes possible to obtain readout image signals with a good signal to noise ratio.
In addition, if the radiation image data recording/reading system of the present invention utilizes an anisotropic stimulable phosphor sheet that restricts the spreading of the excitation light and/or the phosphorescent light within said sheet as its stimulable phosphor sheet, an effect of improving the image quality of the readout image is obtained.
That is, if the spreading of the excitation light within the sheet is suppressed, blurring of the readout image, due to diffused excitation light irradiating a portion of the sheet other than a specified scan area and causing phosphorescent light to be emitted therefrom, is prevented. Further, if spreading of the phosphorescent light within the sheet is suppressed, even if a line sensor which is formed with an extremely thin light receiving surface or the like is utilized, it becomes possible to guide said emitted phosphorescent light efficiently to said light receiving surface, thereby improving the phosphorescent light detection efficiency which in turn improves the image quality of the readout image represented by the output signals of the photoelectric detection means.
Further, in the case that the radiation image data reading apparatus of the present invention is equipped with: a cassette holding portion for receiving a plurality of cassettes in a row; and a shift means for shifting the relative position between the sheet conveyance means, the excitation light main scan means as well as the photoelectric detection means and the aforementioned plurality of cassettes received in said cassette holding portion; wherein by shifting said relative position, it becomes possible to read out radiation image data from each of the stimulable phosphor sheets sequentially conveyed from said plurality of cassettes, the need to remove and install cassettes one by one from and to the apparatus when it is desired to read out radiation image data from each of a plurality of cassettes is obviated. That is, it becomes possible to read out radiation image data from each stimulable phosphor sheet housed within a plurality of cassettes all together, thus realizing a more efficient readout process.